Support is requested for the development of MASDA, Multi-element, Amorphous Silicon Detector Array, the first highly radiation-damage-resistant, solid- state imager for real time portal localization and verification in megavoltage photon radiation therapy. The MASDA imager will consist of a thin, flat panel having a ~25.6x25.6 cm2 imaging surface with lens of thousands of individual sensors and thin film transistors (TFT's) made from a relatively new class of solid-state materials, hydrogenated amorphous silicon, whose properties are uniquely suited to the constraints of radiation therapy imaging. The MASDA panels, in conjunction with other image processing hardware and software, serve to measure the relative fractions of primary photons emitted from a treatment machine (3 to 50 MV) that pass unattenuated through different thicknesses of the irradiated portion of the patient, thereby allowing x ray-like images to be acquired as the patient is being treated. The entire imaging surface is sensitive to the radiation at all times and can be read out after every burst. Consequently, MASDA will produce images of the patient at a rate limited only by the number of available high-energy photon quanta. This, along with the ability to monitor the beams hundreds of times a second, will make this imager a valuable tool in the effective and safe delivery of photon therapy. After computer enhancement and superposition of this imaging information on other treatment planning images, errors in patient-treatment beam alignment can be quickly identified. This information will assist in minimizing geographic misses of the tumor and allow more sophisticated, precise, and aggressive treatment techniques to be developed in anatomically sensitive and geometrically difficult locations. Patient safety will be increased and therapy-room will be used more economically. Three important clinical objectives will be thereby facilitated: (1) assurance that the patient is correctly positioned on the treatment table before the start of the treatment; (2) safeguard against patient motion or machine malfunction during treatment; and (3) documentation of treatment accuracy. As a result of this project, it will be possible to construct a full-size imaging devise with a ~50x50 cm2 detection surface using the prototype MASDA sensor panels.